Although most algae are free living, some instead form symbiotic relationships with other organisms. A symbiotic relationship is one in which two unrelated, or very distantly related, species form an intimate associated the mutually benefits both organisms. Organisms that are in a symbiotic relationship usually evolved together, making use of the other organisms in their environment. The result of many of these symbiotic relationships is that through photosynthesis the algae provide their host with "food" while the host provides the algae with some type of protection.
Modern coral reefs that occupy the world's ocean would not be possible without the algae that live within the coral. This type of algae is called Zooxanthaellae and like most species of algae they are able to use the sunlight through photosynthesis to produce their own food. The presence of these organisms benefits the coral in two ways. First the organic matter produced by the algae is the main source of sustenance for the coral. This allows coral to grow in areas where food would not otherwise be available. Also the photosynthesis of the algae allows the coral to grow at a much faster rate in the light. The number of algae that live in the polyps of the coral can reach nearly 1 million. The benefits to the algae are that it can thrive without living in danger of being eaten and the coral provides them with its waste products.
Lichens also use algae in a symbiotic relationship, sometimes even more so than coral does. In this relationship, much like that of the coral, the algae produces food from the sunlight for the host organisms. Lichens usually house either green algae or a cynobacterium. There are also several lichens that house both types, though in different areas. Although the algae that lives within the lichen can live on its own the lichen cannot, needing the algae to provide a steady stream of food. Algae grow within the hyphae of the lichen as a mass of green cells.
Another close relationship is that of green algae and sponges. An alga only forms a symbiotic relationship with a sponge when it is in enough light for the algae to go through the process of photosynthesis. The algae go through the process of photosynthesis and contribute a large part of their energy toward the sponge that then uses it to speed up their growth. These algae are very important for the sponges energy requirements.